Polystyrene block copolymers containing triethoxysilyl (TES) groups were synthesized by a living anionic polymerization and the subsequent functional group transformation reactions. Then thin polymer films were applied for the reaction with organic acid to form Si-O bonds at the film surface. Surfaceconcentration of Si was confirmed by X-ray photoelectron spectroscopy (XPS) measurement for the resulting polymer film. In addition, contact angle of water droplet for the film increased after acid treatment.These results clearly showed that the polysiloxane substrate was prepared at the outer most surface by treatment with organic acid.Key Words : living anionic polymerization, triethoxysilyl group, surface characterization, block copolymer